It is 1965, and professor skinner is lecturing in an introductory course. he defines psychology as "the scientific study of behavior."

Observable behaviour

Explanation:

It is 1965, and Professor Skinner is lecturing in an introductory course. He defines psychology as "the scientific study of observable behavior."

Professor B.F. Skinner is an American psychologist, he defined psychology as the study of observable behaviour.

d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."